Identical Apertures Research Articles

Characterization of measurement system effects in ultrasonic scattering experiments

A general model is developed to characterize the effect of an ultrasonic measurement system on the experimental determination of ultrasonic scattering as a function of angle and frequency. The model includes arbitrary emitter beams and detector apertures as angular spectra of plane waves. Arbitrary emitted pulses and detector time gates are incorporated through frequency spectra of temporal harmonics. A transformation of variables is employed to express the spectrum of the measured pressure as a product in wave space of a system function and the Fourier transform of the medium variations. The mean-square value of the measured pressure spectrum is similarly expressed as a product of the squared magnitude of the system function and the power spectrum of the medium variations. The measured quantities are shown to become scaled values of intrinsic scattering characteristics when the system function weight is concentrated relative to the medium characteristics in wave space. The assumption of an indefinitely long detector gate is used to represent the system function as a product in which one factor is a beam function dependent on spatial frequency and the other factors are dependent on temporal frequency. Beam-function calculations as well as calculations of second moments and overall beam weight are made for identical Gaussian-shaped emitter and detector apertures to illustrate the blurring and weighting effects of measurement system beam patterns as a function of scattering angle. The moment calculations are shown to identify circumstances when the medium variation function can be factored out from under the integral and the measurement represented as a simple product of the medium properties and a measurement system weight. The results may be used to design scattering experiments in which degradations due to system effects are within acceptable limits.

Both E‐ and H‐plane converging lens applicator for deep and localized microwave hyperthermia

AbstractIn hyperthermia which has recently been considered as a promising cancer therapy, heating of tissue is produced by dielectric heating by microwave energy. On the other hand, owing to the substantial attenuation of electromagnetic waves within a biological body, it is not possible to heat a cancer located deep inside the body. The authors have proposed an antenna (applicator) for electromagnetic irradiation of the biological body with a converging effect in the magnetic field plane (H‐plane). In this paper, a new applicator is proposed which has a converging effect in the electric field plane (E‐plane) in addition to the previous effect in the H‐plane. Such an E‐ and H‐plane converging lens applicator has been fabricated. In addition to the theoretical analysis, a heating experiment has been performed for a phantom model of human muscle. The theoretical and experimental results agree well. If a surface cooling device is added in the applicator, the depth of the maximum temperature point is found to be more than twice that of the conventional waveguide with an identical aperture size. A similar theory has been used at 2450 MHz in the simulation of heating of a biological model considering the presence of cancer. Although the skin depth of the muscle layer at this frequency is 17 mm, the range of heating in the hyperthermia reached more than 30 mm (about twice the surface skin depth), indicating the effectiveness of the applicator developed here.

Phased-array performance degradation due to mirror misfigures, piston errors, jitter, and polarization errors

The purpose of this paper is to develop relationships between the normalized far-field peak intensity (Strehl ratio) and system perturbations, such as mirror misfigure, piston errors, jitter, and polarization errors, for phased-array transmitters. Some of these sources of degradation are unique to phased arrays, whereas others are shared in common with single-aperture transmitters. It was found that for a phased array with many apertures, piston errors must be controlled to within ~λ/20 to ensure a Strehl ratio of 0.9. However, if adaptive optics is used to adjust the path lengths optimally in the free-space regions, then the gain lengths need only be controlled to within λ/(20δn) for a master-oscillator power-amplifier system. The factor δn = (δn/δk)δk is simply the variation of the gain’s index of refraction within one standard deviation in the wave number of the lasing lines. The results found for the effects of jitter are valid for arbitrary-shaped apertures as well as for arbitrary array patterns. For identical apertures and uncorrelated jitter among the apertures, the Strehl ratio is independent of the number of apertures and their arrangement. On the other hand, for correlated jitter, the far-field degradation is sensitive to the array pattern and is least sensitive for a close-packed array. Both the case of isolated perturbations for one element of the array and that of random errors in all the elements of the array are studied.

Electromagnetic transmission through apertures in a cavity in a thick conductor

The problem of electromagnetic coupling from one region to another through an aperture-to-cavity-to-aperture system is specialized to the case of electrically small circular apertures on the axis of a cylindrical cavity of circular cross section for the transverse electromagnetic (TEM) and transverse electric (TE) cases. A simple equivalent circuit for the coupling system is developed. It is found that for certain cavity depths an exceptionally large amount of electromagnetic energy can be transmitted, and that for identical apertures the transmission cross section of the system is independent of the common radius of the apertures as well as the radius of the cavity.

Fringes generated in a Fresnel plane by photographic recordings of identical and laterally shifted speckle patterns

We describe a method to increase and to reverse the contrast of the fringes generated in a Fresnel plane by a photographic plate H, multiple exposed to a random intensity distribution and laterally shifted through ζ H between the exposures. H is limited by two identical apertures separated through 2ζ 0. It is demonstrated that the contrast of the fringes is a function of ζ H and ζ 0. The cases of the two-beam fringes and of the multiple-beam fringes will be considered.

Efficient wideband guided-wave acoustooptic Bragg diffraction using phased surface acoustic wave array in LiNbO_3 waveguides

Efficient wideband guided-wave acoustooptic Bragg diffraction has been demonstrated using a phased surface acoustic wave array in Y-cut LiNbO(3) waveguides. The results of measurement made on the devices which employ the first-order acoustic beam steering from six-element phased-SAWs of relatively small total acoustic aperture, at the center frequency of 325 MHz, have shown that accurate tracking of the Bragg condition is achievable for a frequency band of more than 250 MHz. In one of the deflectors that employ a larger total acoustic aperture, only 68 mW of electric drive power or 3.5 mW of acoustic power was required to diffract 50% of the light over a bandwidth of 112 MHz. This bandwidth is a nearly sixfold increase over that of the deflector that employs a single SAW of identical aperture. The quality of both deflected and undeflected light beams was very good.

Comparison of sonar system performance achievable using synthetic-aperture techniques with the performance achievable by more conventional means

A comparison is made of the signal-to-noise characteristics of synthetic-aperture sonar and conventional sonar. For the synthetic-aperture sonar case a design procedure is described which makes possible the choice of essentially any unambiguous range and any resolution. Often multiple beam receivers are required. It is shown for the synthetic-aperture sonar case that the horizontal aperture is the major factor affecting area coverage rate. The comparisons are made with as many parameter values as spossible identical for both the synthetic-aperture cases and the nonsynthetic-aperture cases. If identical physical aperture dimensions are used and other parameters held to identical values, the synthetic-aperture sonar gives higher signal-to-ratio than does the nonsynthetic-aperture sonar primarily because the latter must operate at higher acoustic frequencies for which signal attenuation becomes extremely large. If the sonar systems are operated at the same frequency, then the nonsynthetic aperture sonar can give a higher signal-to-noise ratio only when its physical aperture is larger than that used in the synthetic-aperture case. Subject Classification: 30.82; 60.20.

Microwave Properties of Thin Films with Apertures

The theory of microwave transmission through thin-film screens of finite conductivity which contain circular, elliptical, and rectangular apertures and which are placed in the transverse plane of an X-band rectangular waveguide is developed. The theory is an extension of Bethe's work in which only screens with infinite conductivity were considered. Experimental verification of the theory is achieved by use of vacuum deposited thin-film screens, and the results are compared with the transmission properties of identical screens of thick foils of the same material. Significant differences are noted in the real and imaginary components of impedance presented by the thin-film and the foil screens with identical apertures. The finite conductivity of the screen is low enough to support an appreciable tangential component of electric field at the film surface.

The preparation of palladium foil objective aperture diaphragms with increased contamination resistance

A new type of foil objective aperture diaphragm for the electron microscope is described. The aperture which is a double layer of palladium and gold, has successfully been made with a total thickness down to approx. 0.4μm of which the gold layer, which is required for extra strength during stripping, only represented a thickness of approx. 0.1μm. The total life span of this palladium aperture was 7–800 hours of beam current irradiation before any axial astigmatism was observed, and approximately twice the life span of gold foil apertures. The optimum conditions for preparing the vacuum evaporated foil apertures are described. In order to increase the reproducibility of foil thicknesses, both for identical and different aperture materials, an evaporation nomogram has been made.

Diffraction Plates for Classroom Demonstrations

The diffraction plate, which consists of an array of many identical apertures or obstacles, produces large Fraunhofer diffraction patterns of sufficiently high intensity such that they may be seen easily with the unaided eye when used with conventional sources. If a small, bright source is observed through a diffraction plate held close to the eye, the Fraunhofer pattern of the apertures appears around the image of the source. If the apertures are randomly distributed over the plate, the pattern produced is characteristic of any one of the apertures in the plate. The pattern produced by a regular array has characteristics of both the individual aperture and of the arrangement. The beautifully colored patterns produced by a white light source are easily seen and photographed. This article describes the computer generation of random and regular arrays of apertures and their applications for classroom demonstrations of the Fraunhofer patterns produced by simple and complex apertures, Babinet's principle, resolution according to the Rayleigh criterion, and many other important aspects of diffraction.

Fraunhofer diffraction pattern of a random distribution of identical apertures in a plane screen

A general expression is derived for the expectation value of the intensity in the Fraunhofer diffraction pattern of a plane screen of any shape containing any number of randomly positioned identical apertures. The special case of a rectangular screen is treated in more detail and the result is generalized for a screen of any shape. It is shown that the expectation value of the intensity has an intense narrow maximum in the centre, the shape of which resembles the Fraunhofer pattern of an aperture of the same size and shape as the screen. The effect of apertures overlapping is taken into account and it is shown that the effect can be corrected for by multiplying two terms in the expression for the expectation value of the intensity by different factors which depend on the number of apertures and the ratio of the area of one aperture to the area of the screen. Consideration is given to the failure of the scalar theory for apertures whose edges are separated by less than one wavelength and it is shown that in most cases the effects due to this cause are less serious than those due to overlapping except where the apertures are smaller than about twice the wavelength.

Use of focused antenna for ionized trail measurements: Part I. Power transfer between two focused antennas

The transmission efficiency for power transmission between two identical circular apertures is considered. Each aperture is focused in a common focal plane which is located midway between them. The transmission efficiency is evaluated in terms of the percentage of the total energy flow through the focal plane which flows through a circular area of arbitrary radius, centered at the focal point and coincident with the focal plane. The results are used to estimate the diffraction error (leakage of energy around the phenomena to be observed) that arises when the focused beam system is used to probe a cylindrical ionized column in the focal plane. Kirchoff theory is used throughout, and perturbation of the focal field distribution due to the presence of the plasma is neglected. The effect of this perturbation has been analyzed theoretically and will be the subject of future papers.

The matching of Pierce guns to tunnels

All Pierce guns which shoot the maximum electron current through two identical apertures collinear with the axis of the gun have a unique ratio of cathode to anode radius. The value of this unique ratio is given for four important cases. A simple design procedure and relevant numerical data are given.

Measured performance of matched dielectric lenses

Two microwave lenses whose surfaces have been matched respectively by embedded capacitive walls and by simulated quarter-wave transformers were built and compared experimentally to an unmatched lens of identical aperture and focal length. At the design frequency, the matched lenses exhibited reductions in side-lobe level of 14 and 10 db with respect to the unmatched lens, and increases in over-all gain of 0.35 and 0.1 db. The input vswr of the feed horn was reduced from 1.6 for the unmatched case to 1.02 and 1.05 for the matched cases. The bandwidth of good performance for the two lenses ranged from lö per cent to at least 44 per cent